fix(planning): never overwrite an incomplete plan without asking (#518)

When /plan runs for new work while tasks/plan.md and tasks/todo.md
still hold unchecked tasks, the agent silently replaces them,
destroying the in-flight plan's state (reported in #518).

Add a convention-neutral guard: replanning the same work updates in
place; different work stops and asks the user before touching the
existing files. Mirror the rule for external task list targets (no
bulk-closing another plan's items), and add matching Rationalization,
Red Flag, and Verification entries plus the guard line in the /plan
command.

Deliberately does not introduce a parallel-lane layout — that
convention decision is being held open in #518; this fixes only the
data-loss half.

.claude/commands/plan.md

@@ -14,3 +14,5 @@ Read the existing spec (SPEC.md or equivalent) and the relevant codebase section
 6. Present the plan for human review
 
 Save the plan to tasks/plan.md and task list to tasks/todo.md.
+
+If tasks/plan.md or tasks/todo.md already exists with unchecked tasks for different work, stop and ask before writing — never silently overwrite an incomplete plan.

skills/planning-and-task-breakdown/SKILL.md

@@ -147,6 +147,13 @@ If a task is L or larger, it should be broken into smaller tasks. An agent perfo
 
 Create the `tasks/` directory if it does not exist.
 
+**Never overwrite an incomplete plan.** Before writing `tasks/plan.md` or `tasks/todo.md`, check whether they already exist and still contain unchecked tasks:
+
+- Same work being replanned (the user asked to revise or extend this plan) → update the existing files in place.
+- Different work → **stop and ask.** The unchecked tasks may be mid-build in another session. Do not delete, overwrite, or rename the existing files on your own; present the conflict and let the user decide (finish the old plan first, explicitly discard it, or tell you where the new plan should go).
+
+The same rule applies to an external task list target: never bulk-close or delete another plan's open tracker items to make room for new ones.
+
 ### Task List Target
 
 The task list target is where tasks and checkpoints are recorded. It is defined once, here; every other reference in this skill defers to it.
@@ -219,10 +226,12 @@ When multiple agents or sessions are available:
 | "The tasks are obvious" | Write them down anyway. Explicit tasks surface hidden dependencies and forgotten edge cases. |
 | "Planning is overhead" | Planning is the task. Implementation without a plan is just typing. |
 | "I can hold it all in my head" | Context windows are finite. Written plans survive session boundaries and compaction. |
+| "The old `tasks/plan.md` is stale, I'll just replace it" | Unchecked tasks may be mid-build in another session. Overwriting them destroys work state that exists nowhere else. Stop and ask. |
 
 ## Red Flags
 
 - Starting implementation without a written task list
+- Overwriting a `tasks/plan.md` or `tasks/todo.md` that still has unchecked tasks for different work, without asking
 - Writing `tasks/todo.md` when the project has designated an external tracker (or scattering tasks across both)
 - Tasks that say "implement the feature" without acceptance criteria
 - No verification steps in the plan
@@ -238,6 +247,7 @@ Before starting implementation, confirm:
 - [ ] Every task has a verification step
 - [ ] Task dependencies are identified and ordered correctly
 - [ ] Tasks are recorded in the task list target (default `tasks/todo.md`)
+- [ ] No pre-existing incomplete plan was overwritten without explicit user confirmation
 - [ ] No task touches more than ~5 files
 - [ ] Checkpoints exist between major phases
 - [ ] The human has reviewed and approved the plan